Method and apparatus for performing model-based layout conversion for use with dipole illumination

ABSTRACT

A method of generating complementary masks for use in a multiple-exposure lithographic imaging process. The method includes the steps of: identifying a target pattern having a plurality of features comprising horizontal and vertical edges; generating a horizontal mask based on the target pattern; generating a vertical mask based on the target pattern; performing a shielding step in which at least one of the vertical edges of the plurality of features in the target pattern is replaced by a shield in the horizontal mask, and in which at least one of the horizontal edges of the plurality of features in the target pattern is replaced by a shield in the vertical mask, where the shields have a width which is greater that the width of the corresponding feature in the target pattern; performing an assist feature placement step in which sub-resolution assist features are disposed parallel to at least one of the horizontal edges of the plurality of features in the horizontal mask, and are disposed parallel to at least one of the vertical edges of the plurality of features in the vertical mask, and performing a feature biasing step in which at least one of the horizontal edges of the plurality of features in the horizontal mask are adjusted such that the resulting feature accurately reproduces the target pattern, and at least one of the vertical edges of the plurality of features in the vertical mask are adjusted such that the resulting feature accurately reproduces the target pattern.

FIELD OF THE INVENTION

The present invention relates to photolithography, and in particularrelates to the generation of mask layouts for use with double dipoleillumination techniques that utilizes a model-based conversion processfor generating the horizontal and vertical mask layouts associated withdipole illumination. In addition, the present invention relates to adevice manufacturing method using a lithographic apparatus comprising aradiation system for providing a projection beam of radiation; a masktable for holding a reticle, serving to pattern the projection beam; asubstrate table for holding a substrate; and a projection system forprojecting the patterned projection beam onto a target portion of thesubstrate.

BACKGROUND OF THE INVENTION

Lithographic projection apparatus (tools) can be used, for example, inthe manufacture of integrated circuits (ICs). In such a case, the maskcontains a circuit pattern corresponding to an individual layer of theIC, and this pattern can be imaged onto a target portion (e.g.comprising one or more dies) on a substrate (silicon wafer) that hasbeen coated with a layer of radiation-sensitive material (resist). Ingeneral, a single wafer will contain a whole network of adjacent targetportions that are successively irradiated via the projection system, oneat a time. In one type of lithographic projection apparatus, each targetportion is irradiated by exposing the entire mask pattern onto thetarget portion in one go; such an apparatus is commonly referred to as awafer stepper. In an alternative apparatus—commonly referred to as astep-and-scan apparatus—each target portion is irradiated byprogressively scanning the mask pattern under the projection beam in agiven reference direction (the “scanning” direction) while synchronouslyscanning the substrate table parallel or anti-parallel to thisdirection; since, in general, the projection system will have amagnification factor M (generally<1), the speed V at which the substratetable is scanned will be a factor M times that at which the mask tableis scanned. More information with regard to lithographic apparatus ashere described can be gleaned, for example, from U.S. Pat. No.6,046,792, incorporated herein by reference.

In a manufacturing process using a lithographic projection apparatus, amask pattern is imaged onto a substrate that is at least partiallycovered by a layer of radiation-sensitive material (resist). Prior tothis imaging step, the substrate may undergo various procedures, such aspriming, resist coating and a soft bake. After exposure, the substratemay be subjected to other procedures, such as a post-exposure bake(PEB), development, a hard bake and measurement/inspection of the imagedfeatures. This array of procedures is used as a basis to pattern anindividual layer of a device, e.g. an IC. Such a patterned layer maythen undergo various processes such as etching, ion-implantation(doping), metallization, oxidation, chemo-mechanical polishing, etc.,all intended to finish off an individual layer. If several layers arerequired, then the whole procedure, or a variant thereof, will have tobe repeated for each new layer. Eventually, an array of devices will bepresent on the substrate (wafer). These devices are then separated fromone another by a technique such as dicing or sawing. Thereafter, theindividual devices can be mounted on a carrier, connected to pins, etc.Further information regarding such processes can be obtained, forexample, from the book “Microchip Fabrication: A Practical Guide toSemiconductor Processing”, Third Edition, by Peter van Zant, McGraw HillPublishing Co., 1997, ISBN 0-07-067250-4, incorporated herein byreference.

The lithographic tool may be of a type having two or more substratetables (and/or two or more mask tables). In such “multiple stage”devices the additional tables may be used in parallel, or preparatorysteps may be carried out on one or more tables while one or more othertables are being used for exposures. Twin stage lithographic tools aredescribed, for example, in U.S. Pat. No. 5,969,441 and WO 98/40791,incorporated herein by reference.

The photolithography masks referred to above comprise geometric patternscorresponding to the circuit components to be integrated onto a siliconwafer. The patterns used to create such masks are generated utilizingCAD (computer-aided design) programs, this process often being referredto as EDA (electronic design automation). Most CAD programs follow a setof predetermined design rules in order to create functional masks. Theserules are set by processing and design limitations. For example, designrules define the space tolerance between circuit devices (such as gates,capacitors, etc.) or interconnect lines, so as to ensure that thecircuit devices or lines do not interact with one another in anundesirable way.

Of course, one of the goals in integrated circuit fabrication is tofaithfully reproduce the original circuit design on the wafer (via themask). Another goal is to use as much of the semiconductor wafer realestate as possible. As the size of an integrated circuit is reduced andits density increases, however, the CD (critical dimension) of itscorresponding mask pattern approaches the resolution limit of theoptical exposure tool. The resolution for an exposure tool is defined asthe minimum feature that the exposure tool can repeatedly expose on thewafer. The resolution value of present exposure equipment oftenconstrains the CD for many advanced IC circuit designs.

Furthermore, as the demand continues for even higher performance ofsemiconductor devices, the design rule shrink rate is outpacing theprogress of both of the exposure wavelength reduction and theadvancement of high Numerical Aperture (NA) lenses. This factor haspresented a challenge to lithographers to push optical lithographybeyond the limit that was thought possible a few years ago. As is known,Resolution Enhancement Techniques (RETs) have become indispensable inlow k₁ optical lithography. Strong Off-Axis Illumination (OAI), whichuses 2-beam imaging with symmetrical 0^(th) and 1^(st) orders in thelens pupil, can greatly enhance resolution and contrast. Dipoleillumination is the most extreme case of OAI, and is capable ofproviding better imaging contrast with improved process latitude forvery low k₁ imaging.

However, one of the limitations associated with dipole illumination isthat a single illumination only enhances resolution for features thatare orthogonal to the illumination pole axis. As a result, in order totake full advantage of dipole illumination during wafer printing, themask pattern must be converted into horizontal and verticalorientations. Once the mask pattern is converted in this manner, aY-pole exposure is utilized to image the horizontally oriented features,and a X-pole exposure is utilized to image the vertically orientedfeatures. One important aspect of dipole illumination is that whenimaging the horizontally oriented features, the vertically orientedfeatures must be protected (i.e., shielded) so the vertically orientedfeatures are not degraded. The opposite is true when vertically orientedfeatures are imaged (i.e., the horizontally oriented features must beprotected).

FIG. 1 illustrates the basic concepts of double dipole imaging. Asstated, typically there are at least two exposures when utilizing dipoleillumination. In the first exposure, the X dipole aperture 10 provides amaximum aerial image intensity (i.e., maximum modulation) for thevertical portion of the lines 12 to be printed. The resulting imageprofile is illustrated by line 24 in FIG. 1. In the second exposure,which utilizes the Y-dipole aperture 16, there is no image modulationfor lines 12. It is noted, however, that during the second exposureusing the Y-dipole aperture, the vertical portions of the lines 12 needto be shielded so that the vertical features formed during the firstexposure are not degraded during the second exposure. FIG. 1 illustratesshielding the lines 12 with shields 15, each of which is formed byenlarging the target line 12 in the horizontal direction 20 nm per side.It is noted that the amount of shielding corresponds to the amount thatthe vertical edges of the feature are extended. For example, assuming aninitial line width of 70 nm, each vertical edge would be extended anadditional 20 nm, which would result in a total width of shield 15 of110 nm. As a result of the shielding, when exposing the horizontal linesusing the Y dipole aperture, there is substantially no imaging (i.e.,modulation) of the vertical features 12. The aerial image is a DCcomponent as shown by line 17 in FIG. 1, which corresponds to the 20 nmshielding. The final aerial image intensity, which is represented byline 14 in FIG. 1, corresponds to the sum of the first exposure usingthe X dipole aperture and the second exposure using the Y dipoleaperture.

It is further noted that, assuming the exposure energy is constant,increasing the width of the shielding of each edge from a 20 nm shield15 to a 40 nm shield 20 for the vertical lines 12 causes the minimalintensity level of the resulting image to shift to a lower level. Thisis represented by line 22 in FIG. 1, which represents the aerial imageassociated with the vertical portions of the features. As shown, theaerial image 22 is just a DC component. However, it is lower than the DCcomponent 17 associated with the 20 nm shield. As a result, thecomposite image 19 formed utilizing the 40 nm shielding provides betterimaging results than the composite image 14 formed utilizing the 20 nmshielding.

When utilizing dipole illumination techniques, as a result of the needto separate the horizontally and vertically oriented features, one ofthe challenges for the lithographer is determining how to convert theoriginal IC design data into its horizontal or vertical patterncomponents and generate two masks for the dual exposure process that cantake full advantage of the dipole imaging performance.

FIG. 2 is a flowchart which depicts an exemplary rule-baseddecomposition process for generating the horizontal and vertical masksbased on a target design. Referring to FIG. 2, the first step (Step 200)is to decompose or separate the original layout 19 (i.e., target design)into primarily horizontal features and vertical features. The next step(Step 220) is to apply shielding to both masks, using rule-based edgetreatment. Typically, the shielding rules are empirical determined(e.g., the rules are defined using experimental results at a givennumber of pitch intervals). The third step (Step 240) is to applyscattering bars (i.e., OPC technique) to both dipole masks (horizontaland vertical). Specifically, after the shielding has been applied in thefirst step, scattering bar OPC is then applied to each of theorientations. The scattering bars (SB) are applied only along theorientations perpendicular to the pole (i.e., vertical SBs on thevertical mask and horizontal SBs on the horizontal mask). The fourthstep (Step 260) is to provide negative serif treatment (also known aswiden/deepen) to concave corners. The purpose is to provide cuts-outs inthe shielding, near concave corners, which allow enough light to definethe edges along the high modulation orientation, causing betterline-definition in the concave corners. The final step (Step 280) inthis approach is to apply fine biasing along the critical highmodulation orientations for each mask. The horizontal mask will havefine biasing treatment along the horizontal edges and the vertical maskwill have the fine biasing along the vertical edges.

Currently known techniques for generating the horizontal and verticalmasks do not always produce optimal results when attempting to generatesuch masks for complex structures. For example, utilizing knowntechniques such as the one disclosed in the flowchart of FIG. 2, it issometimes difficult, for example, to provide proper shielding forcomplex structures, such as, “jogs” (i.e., short changes in either thevertical or horizontal direction, for example, a short vertical step,between two long horizontal lines), short “S” turns and/or U-shapedpatterns in the design. Moreover, it is sometimes difficult to determinewhether or not a given structure of the target design should be treatedas a horizontal structure or a vertical structure when initiallygenerating the horizontal and vertical masks. As a result, it was oftennecessary to have an experienced mask designer involved with thegeneration of the horizontal and vertical masks for addressing andrectifying the foregoing issues in order to generate acceptable maskdesigns.

Accordingly, there exists a need for a method which allows for thegeneration of both horizontal and vertical masks utilized in conjunctionwith double dipole imaging, which eliminates the issues anddisadvantages associated with prior art techniques for generatinghorizontal and vertical masks.

SUMMARY OF THE INVENTION

In an effort to solve the foregoing needs, it is one object of thepresent invention to provide a method for generating the horizontal andvertical masks for performing dipole illumination which simplifies thelayout conversion process relative to the prior art techniques andallows for a more straight-forward design approach.

More specifically, the present invention relates to a method ofgenerating complementary masks for use in a multiple-exposurelithographic imaging process. The method includes the steps of:identifying a target pattern having a plurality of features comprisinghorizontal and vertical edges; generating a horizontal mask based on thetarget pattern; generating a vertical mask based on the target pattern;performing a shielding step in which at least one of the vertical edgesof the plurality of features in the target pattern is replaced by ashield in the horizontal mask, and in which at least one of thehorizontal edges of the plurality of features in the target pattern isreplaced by a shield in the vertical mask, where the shields have awidth which is greater that the width of the corresponding feature inthe target pattern; performing an assist feature placement step in whichsub-resolution assist features are disposed parallel to at least one ofthe horizontal edges of the plurality of features in the horizontalmask, and are disposed parallel to at least one of the vertical edges ofthe plurality of features in the vertical mask, and performing a featurebiasing step in which at least one of the horizontal edges of theplurality of features in the horizontal mask are adjusted such that theresulting feature accurately reproduces the target pattern, and at leastone of the vertical edges of the plurality of features in the verticalmask are adjusted such that the resulting feature accurately reproducesthe target pattern.

Although specific reference may be made in this text to the use of theinvention in the manufacture of ICs, it should be explicitly understoodthat the invention has many other possible applications. For example, itmay be employed in the manufacture of integrated optical systems,guidance and detection patterns for magnetic domain memories,liquid-crystal display panels, thin-film magnetic heads, etc. Theskilled artisan will appreciate that, in the context of such alternativeapplications, any use of the terms “reticle”, “wafer” or “die” in thistext should be considered as being replaced by the more general terms“mask”, “substrate” and “target portion”, respectively.

In the present document, the terms “radiation” and “beam” are used toencompass all types of electromagnetic radiation, including ultravioletradiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm) andEUV (extreme ultra-violet radiation, e.g. having a wavelength in therange 5–20 nm).

The term mask as employed in this text may be broadly interpreted asreferring to generic patterning means that can be used to endow anincoming radiation beam with a patterned cross-section, corresponding toa pattern that is to be created in a target portion of the substrate;the term “light valve” can also be used in this context. Besides theclassic mask (transmissive or reflective; binary, phase-shifting,hybrid, etc.), examples of other such patterning means include:

a) A programmable mirror array. An example of such a device is amatrix-addressable surface having a viscoelastic control layer and areflective surface. The basic principle behind such an apparatus is that(for example) addressed areas of the reflective surface reflect incidentlight as diffracted light, whereas unaddressed areas reflect incidentlight as undiffracted light. Using an appropriate filter, the saidundiffracted light can be filtered out of the reflected beam, leavingonly the diffracted light behind; in this manner, the beam becomespatterned according to the addressing pattern of the matrix-addressablesurface. The required matrix addressing can be performed using suitableelectronic means. More information on such mirror arrays can be gleaned,for example, from United States Patents U.S. Pat. No. 5,296,891 and U.S.Pat. No. 5,523,193, which are incorporated herein by reference.

b) A programmable LCD array. An example of such a construction is givenin United States Patents U.S. Pat. No. 5,229,872, which is incorporatedherein by reference.

The method of the present invention provides important advantages overthe prior art. Most importantly, instead of using a complicatedhand-crafted rule-based decomposition flow, the method of the presentinvention allows for a simplified model-based procedure for generatingthe horizontal and vertical masks from the target design.

Further, the method of the present invention not only simplifies theprocessing flow, but it also helps to eliminate several significantproblems associated with known techniques. For example, in comparison tothe present invention, the known rule-based method illustrated in FIG. 2requires the generation of a dedicated rule for each and every kind offeature to be printed. Thus, for any practical design, the set of rulesthat must be generated is prohibitively large. In contrast, with themethod of the present invention, there is no need to build a library ofrules: The table illustrated in FIG. 13 lists some of the differencesbetween the method of the present invention and the known rule-basedmethod.

In addition, the model-based approach of the present invention alsoprovides shielding treatment, which more effectively handles difficultstructures, such as, jogs, S-turns and U-turns. Further, the method ofthe present invention is capable of simultaneously converting complexlogic and memory mask patterns in double dipole lithography (DDL)compatible mask layouts.

Additional advantages of the present invention will become apparent tothose skilled in the art from the following detailed description ofexemplary embodiments of the present invention.

The invention itself, together with further objects and advantages, canbe better understood by reference to the following detailed descriptionand the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the principles of dipole illumination.

FIG. 2 illustrates an exemplary flowchart of a rule-based decompositionmethod for generating horizontal and vertical masks for use with doubledipole illumination.

FIG. 3 is an exemplary flowchart illustrating the model-basedlayout-conversion method of the present invention.

FIGS. 4 a and 4 b illustrate the horizontal mask and vertical mask,respectively, after shielding has been applied in accordance with themethod of the present invention.

FIGS. 4 c and 4 d illustrate the horizontal mask and vertical mask,respectively, after scatter bars have been applied in accordance withthe method of the present invention.

FIGS. 4 e and 4 f illustrate the horizontal mask and vertical mask,respectively, after fine feature biasing has been performed inaccordance with the method of the present invention.

FIG. 5 a illustrates a calculated resist contour with double exposureusing the model-based generated mask layouts illustrated in FIGS. 4 eand 4 f, and FIG. 5 b illustrates the corresponding 3D profile.

FIG. 6 illustrates an exemplary flowchart illustrating the process ofgenerating a SPIF utilized in the model-based OPC method of the presentinvention.

FIGS. 7 a and 7 b illustrate an exemplary horizontal mask and anexemplary vertical mask prior to performing the shielding step of thepresent invention.

FIGS. 8 a and 8 b illustrate exemplary intensity distributionscorresponding to the masks illustrated in FIGS. 7 a and 7 b.

FIGS. 9 a and 9 b illustrate the horizontal mask and the vertical maskafter the shielding step is performed.

FIGS. 10 a and 10 b illustrate exemplary intensity distributionscorresponding to the masks illustrated in FIGS. 9 a and 9 b.

FIGS. 11 a and 11 b illustrate the further modification of thehorizontal and vertical mask so as to provide for fine feature biasing.

FIGS. 12 a and 12 b illustrate exemplary intensity distributionscorresponding to the masks illustrated in FIGS. 11 a and 11 b.

FIG. 13 illustrates a table illustrating exemplary differences betweenthe method of the present invention and the known rule-based method.

FIG. 14 schematically depicts a lithographic projection apparatussuitable for use with the masks designed with the aid of the currentinvention.

DETAILED DESCRIPTION OF THE INVENTION

As explained in more detail below, the preferred embodiment of thepresent invention provides a method and apparatus for generatinghorizontal and vertical masks for use with dipole illumination.Specifically, the method of the present invention relates to amodel-based layout-conversion method for generating the horizontal andvertical masks from a target design.

FIG. 3 is an exemplary flowchart illustrating the model-basedlayout-conversion method of the present invention. As is shown in FIG.3, the model-based approach entails essentially 3 basic processingsteps.

Referring to FIG. 3, the first step (Step 32) of the model-basedapproach comprises applying shielding utilizing a model-based OPC to thevertical edges in the target pattern so as to define the horizontalmask, and applying shielding to the horizontal edges in the targetpattern so as to define the vertical mask. In other words, the startingpoint for both the horizontal mask and the vertical mask is the targetdesign. It is noted that the target design data can be made available inGDSII data format 19, or any other suitable format. Then, in thehorizontal mask, the vertical edges in the target design have shieldingapplied thereto, and in the vertical mask, the horizontal edges in thetarget design have shielding applied thereto. As explained furtherbelow, the amount of shielding to be applied is determined utilizing theOPC model (based on for example, an aerial image or an empirical model).It is also noted that the OPC model could also be based on a theoreticalmodel to determine the shielding amounts to be applied to the targetpattern. It is noted that as explained further below, it is possiblethat not all of the vertical and horizontal edges will have shieldingapplied thereto, and the amount of shielding applied may also vary.

In accordance with the present invention, an OPC model, which isrepresented as a function, that defines the imaging performance ofimaging system (i.e., the imaging tool, the imaging conditions, theresist, the resist processing, etc.) is generated. Once this function,which is referred to as a system pseudo intensity function (SPIF), isgenerated, it can be utilized to automatically adjust/revise the targetpattern to be imaged such that the printed image matches the targetpattern (within some defined error margin). In other words, it is thisSPIF function which is utilized to automatically define the necessaryshielding applied to the horizontal and vertical masks to reproduce thedesired target.

The generation of SPIF functions has been described, for example, inU.S. patent application Ser. No. 10/266,922, which is herebyincorporated by reference. A brief description of the process isprovided. The first step is to determine a set of performance parameterswhich define the printing performance of the given imaging system. Next,in one embodiment, actual images are compared to predicted images formultiple types of features (e.g., densely-spaced features, semi-isolatedfeatures, isolated features, line-ends, elbows, etc.) to determine howfar the printed image (i.e., actual image) deviates from the predictedimage. Then, the performance parameters, defining the printing process,are adjusted such that the predicted image matches the actual imageprinted on the wafer. At this point, the SPIF function with theassociated performance parameters accurately represents the imagingperformance of the given imaging process. An example of a SPIF is asfollows:

${{SPIF}\mspace{11mu}( {x,y} )} = {\sum\limits_{i = 1}^{n}{\alpha_{i}{{{M( {x,y} )}*{\Psi_{i}( {x,y} )}}}^{2}}}$where,

-   α_(i) is a weighting coefficient to be calibrated and optimized;-   M(x, y) is the mask transmission function;-   Ψ_(i)(x, y) is the set of basis functions that have been chosen to    represent optical imaging system, e.g., the Eigen functions of a    theoretical optical-system;-   x, y are the location of wafer pattern; and-   * is the convolution operator.

By utilizing the SPIF to predict how a given design pattern would print,it is possible to optimize the reticle design to print the desired(“target”) pattern. For this type of optimization process, one can useany well-established optimization method, for example, such asLevenberg-Marquardt: In other words, the SPIF can be utilized to definehow features in the target pattern should be modified in the reticle toobtain the target pattern in the imaged substrate.

FIG. 6 illustrates an exemplary flowchart illustrates the foregoingprocess of generating a SPIF utilized in the model-based OPC method ofthe present invention. In the first step, Step 601, an initial SPIFfunction representing the imaging process is defined. In the next step,Step 602, SEM images of actual test patterns imaged on substrates aretaken. In the given embodiment, the test patterns are imaged utilizingdouble dipole imaging so as to accurately reflect the resulting patternafter illumination utilizing a vertical dipole and a horizontal dipole.In Step 609, the SPIF function is initially evaluated utilizing the testpatterns to generated a predicted pattern Next, Step, 603, the predictedimage obtained by utilizing the SPIF function is compared to the actualprinted test patterns to determine the difference between the tworesults. If the difference is below some predefined error criteria, Step605, the SPIF function is acceptable, and is utilized to define printingperformance, Step 606. However, if the difference is not below thepredefined error criteria, the variables in the SPIF function arevaried, Step 607, and a new predicted pattern based on the modified SPIFfunction is computed, Step 609. The new predicted pattern is thencompared to the actual printed test patterns in the same manner as setforth above. This process continues until the SPIF function produces apredicted pattern which matches the actual printed pattern within thepredefined error criteria. It is noted that in the preferred embodiment,a two dimensional contour of the printed test pattern is taken utilizingthe SEM.

As noted above, once the SPIF function is generated, it can be utilizedto determine the amount of shielding to be applied to the edges of thehorizontal and vertical features in the vertical and horizontal masks,respectively. In other words, the SPIF function is utilized toautomatically determine whether or not shielding is required to beapplied to a given feature edge, as well as the amount of shieldingrequired so that the target pattern is accurately reproduced in theimaged substrate.

FIGS. 4 a and 4 b illustrate the process associated with Step 32. FIG. 4a illustrates the horizontal mask after shielding has been applied asdefined by the SPIF function. As shown, the vertical edges in the targetpattern (indicated by the solid lines 41) has shielding 43 appliedthereto. As noted, this shielding 43 is necessary for protecting thevertical edges of the target pattern when imaging the horizontal edgesof the target pattern. Similarly, FIG. 4 b illustrates the vertical maskafter shielding has been applied. As shown, the horizontal edges in thetarget pattern (indicated by the solid lines 41) has shielding 45applied thereto. Again, this shielding 45 is necessary for protectingthe horizontal edges of the target pattern when imaging the verticaledges of the target pattern. As noted, it is possible that not everyhorizontal edge or vertical edge in the respective masks will haveshielding applied thereto. Shielding is applied only where deemednecessary by the OPC model (i.e., SPIF function).

Thus, in accordance with the current method, the calibrated model, whichcorresponds to the SPIF function, defines all shielding edge treatmentto be applied to the target design in both the horizontal and verticalmasks. The output of Step 32 is horizontal and vertical masks withoptimal shielding as applied by the model. It is noted that in contrastto the prior art methods discussed above, when practicing the modelbased method disclosed herein, there is no need to perform the step ofseparating the original design into horizontal and vertical features(which can be a difficult task). Nor is there a need for generatingspecific rules for treating each of the possible features that could beencountered in a practical design. As noted, in Step 32 shielding isapplied on the basis of the edges orientation (i.e., vertical edges areshielded in the horizontal mask, and horizontal edges are shielded inthe vertical mask) in accordance with the SPIF function.

The next step in the process (Step 34) entails the application ofsub-resolution scatter bars or assist features to the horizontal andvertical masks. The result of Step 34 is illustrated in FIGS. 4 c and 4d. More specifically, referring to FIG. 4 c, which illustrates thehorizontal mask, the scatter bars 47 are only disposed adjacent thehorizontal edges to be imaged. Similarly, referring to FIG. 4 d, whichillustrates the horizontal mask, the scatter bars 47 are only disposedadjacent the vertical edges to be imaged. The use of such sub-resolutionscatter bars and assist features to provide OPC is well known by thoseof skill in the art and is taught for example, in U.S. Pat. No.5,821,014, which is hereby incorporated by reference. Accordingly,additional disclosure regarding scatter bars is omitted.

The third and final step in the process (Step 36) entails applying finebiasing along the critical high modulation orientations for both thehorizontal and vertical masks. More specifically, fine biasing 49 isapplied along the horizontal edges in the horizontal mask, and finebiasing is applied to the vertical edges in the vertical mask. FIG. 4(e) illustrates an example of the resulting horizontal mask afterapplication of fine biasing of the horizontal edges to be imaged, andFIG. 4( f) illustrates an example of the resulting vertical mask afterapplication of fine biasing of the vertical edges to be imaged. It isnoted that the fine biasing is automatically performed based on themodel so as to provide an additional level of OPC. Referring to FIGS. 4e and 4 f, it is shown that the edges to be imaged in the givenhorizontal and vertical masks are modified from the straight edgesillustrated in the corresponding horizontal and vertical masksillustrated in FIGS. 4 c and 4 d, respectively. As an example, the modelmay provide, for example, automatically defining the in-cut amount(i.e., deepen amount) in the negative serif area so as to accuratelydefine all edges, or adjustment of the feature edges to provide for CDcontrol. Once the fine biasing step has been performed, the process ofgenerating the horizontal and vertical masks is complete, and the maskscan be utilized to image the wafer.

FIG. 5 a illustrates the calculated/predicted full resist contour withdouble exposure using the full model-based generated mask layoutsillustrated in FIGS. 4 e and 4 f, and FIG. 5 b illustrates thecorresponding 3D profile.

As noted above, the model based OPC utilized in conjunction with thepresent invention is determined based on, for example, an aerial imageor an empirical model, or alternatively, a theoretical model. A moredetailed example of the application of the model based OPC for providingthe shielding required for double-dipole lithography techniques is setforth below. FIGS. 7 a and 7 b illustrate an exemplary target pattern,which is a “T” shaped feature. FIG. 7 a illustrates the horizontal mask(to be imaged utilizing the Y-dipole) and FIG. 7 b illustrates thevertical mask (to by imaged utilizing the X-dipole). Initially, both thehorizontal mask and the vertical mask are identical to the target mask.Then, as explained herein, the shielding is adjusted by the model OPCrule such that the target pattern is accurately reproduced withinacceptable limits.

Referring to FIGS. 7 a and 7 b, in order to illustrate this process, cutlines 71 and 72 are defined at which the intensity distribution can beexamined. The first cut line 71 is taken across a vertical feature, andthe second cut line 72 is taken across a horizontal feature. Anexemplary intensity distribution at cut line 71 is illustrated in FIG. 8a and an exemplary intensity distribution at cut line 72 is illustratedin FIG. 8 b. It is noted that the location of the cut lines and theintensity distributions are merely exemplary. Further, the intensitydistributions illustrated in FIGS. 8 a, 8 b, 10 a, 10 b, 12 a and 12 bare for illustration purposes only and are not intended to represent orcorrespond to actual intensity distributions.

Referring to FIGS. 8 a and 8 b, the intensity distribution following anillumination with a dipole X is labelled DX, and the intensitydistribution following an illumination with a dipole Y is labelled DY.The difference between the intensity distribution of FIG. 8 a and theintensity distribution of FIG. 8 b is the shape of the intensitydistribution following from a DX or a DY illumination. The thresholdprovides the intensity at which the resist develops. As shown in thegiven example, the sum of the two exposures lies above the threshold forboth cut lines. Thus, in this situation, if the masks corresponded tothe original target pattern, no feature will be printed at all. In orderto lower the intensity distribution, the edges of the target feature arebiased outwards (i.e., shielding) in accordance with the shieldingrequirements defined by the model OPC rule. In accordance with thepresent invention, only the vertical edges of the horizontal mask andthe horizontal edges of the vertical mask are biased outwardly (i.e.,shielded). This step corresponds to the shielding step in the process.As a result, the contribution of the “wrong” exposure to the aerialimage is decreased. FIGS. 9 a and 9 b illustrate the horizontal maskhaving its vertical edges shield and the vertical mask having itshorizontal edges shielded, respectively. Referring to FIGS. 10 a and 10b, after applying the shielding, the sum of the two exposures dropsbelow the threshold intensity at some positions so as to allow forprinting of the features.

It is also noted that in FIGS. 7 a, 7 b, 9 a, 9 b, 11 a and 11 b, theshielding is illustrated as being applied uniformly to each edge of thefeature. While such a uniform application is one possibility, it is alsopossible for each given edge to be treated as individual segments andhave shielding applied (or not) on a segment by segment basis. Forexample, a given feature edge could be separated into individualsegments ranging in length, for example, from 20–500 nm, with eachsegment be considered and treated as an individual edge.

FIGS. 11 a and 11 b illustrate the further modification of thehorizontal and vertical mask so as to provide for fine feature biasing.The position interval over which the intensity drops below the thresholdis the line width at the cut line. After the shielding step isperformed, the line width is typically not equal to the targeted linewidth. The fine feature biasing step is utilized to correct for theseoffsets. As shown in FIGS. 11 a and 11 b, the horizontal edges of thefeatures in the horizontal mask and the vertical edges of the featuresin the vertical mask are adjusted such that the line width of thepredicted line is the same as the line width of the targeted line at thegiven location. FIGS. 12 a and 12 b illustrate the intensitydistributions at the cut lines as performing the fine feature biasingstep.

It is noted that variations of the method disclosed above are alsopossible. For example, the model-based placement of shielding structuresis not limited to just double dipole lithography. Multiple exposurelithography can use a variety of illuminators, such as quasar, annular,hexapole and soft dipole (i.e., dipole plus a central spot). Shieldingis often a requirement of multiple exposure lithography. Thus, themodel-based shielding technique of the present invention can also beutilized for all variations of multiple exposure technologies requiringshielding.

In addition, the order of the process of the present invention is notlimited to the specific order illustrated in FIG. 3. For example, it isalso possible for the application of the SBs (Step 34) to be performedprior to the application of the shielding (Step 32).

It is further noted that the foregoing method of the present inventioncan be incorporated into a CAD (computer-aided design) programs, such asdiscussed above, which would function in-part to perform the foregoingmethod and generate files representing the horizontal and vertical masksproduced by the process of the present invention.

The present invention also provides significant advantages over theprior art. Most importantly, instead of using a complicated hand-craftedrule-based decomposition flow, the method of the present inventionallows for a simplified model-based procedure for generating thehorizontal and vertical masks from the target design.

Further, the method of the present invention not only simplifies theprocessing flow, but it also helps to eliminate several significantproblems associated with known techniques. For example, in comparison tothe present invention, the known rule-based method illustrated in FIG. 2requires the generation of a dedicated rule for each and every kind offeature to be printed. Thus, for any practical design, the set of rulesthat must be generated is prohibitively large. In contrast, with themethod of the present invention, there is no need to build a library ofrules. The table illustrated in FIG. 13 lists some of the differencesbetween the method of the present invention and the known rule-basedmethod.

In addition, the model-based approach of the present invention alsoprovides shielding treatment, which more effectively handles difficultstructures, such as, jogs, S-turns and U-turns. Further, the method ofthe present invention is capable of simultaneously converting complexlogic and memory mask patterns in double dipole lithography (DDL)compatible mask layouts.

FIG. 14 schematically depicts a lithographic projection apparatussuitable for use with the masks designed with the aid of the currentinvention. The apparatus comprises:

-   a radiation system Ex, IL, for supplying a projection beam PB of    radiation. In this particular case, the radiation system also    comprises a radiation source LA;-   a first object table (mask table) MT provided with a mask holder for    holding a mask MA (e.g. a reticle), and connected to first    positioning means for accurately positioning the mask with respect    to item PL;-   a second object table (substrate table) WT provided with a substrate    holder for holding a substrate W (e.g. a resist-coated silicon    wafer), and connected to second positioning means for accurately    positioning the substrate with respect to item PL;-   a projection system (“lens”) PL (e.g. a refractive, catoptric or    catadioptric optical system) for imaging an irradiated portion of    the mask MA onto a target portion C (e.g. comprising one or more    dies) of the substrate W.

As depicted herein, the apparatus is of a transmissive type (i.e. has atransmissive mask). However, in general, it may also be of a reflectivetype, for example (with a reflective mask). Alternatively, the apparatusmay employ another kind of patterning means as an alternative to the useof a mask; examples include a programmable mirror array or LCD matrix.

The source LA (e.g. a mercury lamp, excimer laser or plasma dischargesource) produces a beam of radiation. This beam is fed into anillumination system (illuminator) IL, either directly or after havingtraversed conditioning means, such as a beam expander Ex, for example.The illuminator IL may comprise adjusting means AM for setting the outerand/or inner radial extent (commonly referred to as σ-outer and σ-inner,respectively) of the intensity distribution in the beam. In addition, itwill generally comprise various other components, such as an integratorIN and a condenser CO. In this way, the beam PB impinging on the mask MAhas a desired uniformity and intensity distribution in itscross-section.

It should be noted with regard to FIG. 14 that the source LA may bewithin the housing of the lithographic projection apparatus (as is oftenthe case when the source LA is a mercury lamp, for example), but that itmay also be remote from the lithographic projection apparatus, theradiation beam that it produces being led into the apparatus (e.g. withthe aid of suitable directing mirrors); this latter scenario is oftenthe case when the source LA is an excimer laser (e.g. based on KrF, ArFor F₂ lasing). The current invention encompasses both of thesescenarios.

The beam PB subsequently intercepts the mask MA, which is held on a masktable MT. Having traversed the mask MA, the beam PB passes through thelens PL, which focuses the beam PB onto a target portion C of thesubstrate W. With the aid of the second positioning means (andinterferometric measuring means IF), the substrate table WT can be movedaccurately, e.g. so as to position different target portions C in thepath of the beam PB. Similarly, the first positioning means can be usedto accurately position the mask MA with respect to the path of the beamPB, e.g. after mechanical retrieval of the mask MA from a mask library,or during a scan. In general, movement of the object tables MT, WT willbe realized with the aid of a long-stroke module (coarse positioning)and a short-stroke module (fine positioning), which are not explicitlydepicted in FIG. 14. However, in the case of a wafer stepper (as opposedto a step-and-scan tool) the mask table MT may just be connected to ashort stroke actuator, or may be fixed.

The depicted tool can be used in two different modes:

-   In step mode, the mask table MT is kept essentially stationary, and    an entire mask image is projected in one go (i.e. a single “flash”)    onto a target portion C. The substrate table WT is then shifted in    the x and/or y directions so that a different target portion C can    be irradiated by the beam PB;-   In scan mode, essentially the same scenario applies, except that a    given target portion C is not exposed in a single “flash”. Instead,    the mask table MT is movable in a given direction (the so-called    “scan direction”, e.g. the y direction) with a speed v, so that the    projection beam PB is caused to scan over a mask image;    concurrently, the substrate table WT is simultaneously moved in the    same or opposite direction at a speed V=Mv, in which M is the    magnification of the lens PL (typically, M=¼ or ⅕). In this manner,    a relatively large target portion C can be exposed, without having    to compromise on resolution.

Although certain specific embodiments of the present invention have beendisclosed, it is noted that the present invention may be embodied inother forms without departing from the spirit or essentialcharacteristics thereof. For example, the method of the presentinvention can be utilized with all pole angles and sigma settings, andis not intended to be limited to the pole angles and sigma settings setforth and illustrated in the exemplary embodiments herein. The presentembodiments are therefore to be considered in all respects asillustrative and not restrictive, the scope of the invention beingindicated by the appended claims, and all changes that come within themeaning and range of equivalency of the claims are therefore intended tobe embraced therein.

1. A method of generating complementary masks for use in amultiple-exposure lithographic imaging process, said method comprisingthe steps of: generating a model defining the imaging performance ofsaid multiple-exposure lithographic imaging process, identifying atarget pattern having a plurality of features, said plurality offeatures comprising horizontal and vertical edges, generating ahorizontal mask based on said target pattern, generating a vertical maskbased on said target pattern, and performing a shielding step in whichat least one of the vertical edges of the plurality of features in thetarget pattern is replaced by a shield in the horizontal mask, and inwhich at least one of the horizontal edges of the plurality of featuresin the target pattern is replaced by a shield in the vertical mask, saidshields having a width which is greater that the width of thecorresponding feature in the target pattern, wherein the amount ofshielding to be applied in said shielding step is determined utilizingsaid model.
 2. The method of generating complementary masks according toclaim 1, further comprising: performing an assist feature placement stepin which sub-resolution assist features are disposed parallel to atleast one of the horizontal edges of the plurality of features in thehorizontal mask, and are disposed parallel to at least one of thevertical edges of the plurality of features in the vertical mask, andperforming a feature biasing step in which at least one of thehorizontal edges of the plurality of features in the horizontal mask areadjusted such that the resulting feature accurately reproduces thetarget pattern, and at least one of the vertical edges of the pluralityof features in the vertical mask are adjusted such that the resultingfeature accurately reproduces the target pattern.
 3. The method ofgenerating complementary masks according to claim 2, wherein said assistfeature placement step is performed prior to said shielding step.
 4. Themethod of generating complementary masks according to claim 2, whereinthe amount of biasing applying to a given feature edge in said featurebiasing step is determined utilizing said model.
 5. The method ofgenerating complementary masks according to claim 1, wherein prior tosaid shielding step said horizontal mask and said vertical mask areidentical to said target pattern.
 6. The method of generatingcomplementary masks according to claim 1, wherein said model isdetermined utilizing one of a calibrated model of the multiple-exposurelithographic imaging process or a theoretical model of themultiple-exposure lithographic imaging process.
 7. A devicemanufacturing method comprising the steps of: (a) providing a substratethat is at least partially covered by a layer of radiation-sensitivematerial; (b) providing a projection beam of radiation using a radiationsystem; (c) using patterning means to endow the projection beam with apattern in its cross-section; (d) projecting the patterned beam ofradiation onto a target portion of the layer of radiation-sensitivematerial, wherein steps (c) and (d) are performed a first time with afirst pattern and then a second time with a second pattern, said firstand second patterns being generated using a method according to claim 1.8. The method of generating complementary masks according to claim 1,wherein said shielding step comprises dividing a given edge to beshielded into a plurality of segments and having the shielding appliedon a segment by segment basis.
 9. An apparatus for generatingcomplementary mask patterns for use in a multiple-exposure lithographicimaging process, said apparatus comprising: means for generating a modeldefining the imaging performance of said multiple-exposure lithographicimaging process; means for identifying a target pattern having aplurality of features, said plurality of features comprising horizontaland vertical edges, means for generating a horizontal mask based on saidtarget pattern, means for generating a vertical mask based on saidtarget pattern, and means for performing shielding in which at least oneof the vertical edges of the plurality of features in the target patternis replaced by a shield in the horizontal mask, and in which at leastone of the horizontal edges of the plurality of features in the targetpattern is replaced by a shield in the vertical mask, said shieldshaving a width which is greater that the width of the correspondingfeature in the target pattern, wherein the amount of shielding to beapplied to said at least one vertical edge of the plurality of featuresand said at least one horizontal edge of the plurality of features isdetermined utilizing said model.
 10. An apparatus for generatingcomplementary masks according to claim 9, further comprising: means forperforming an assist feature placement in which sub-resolution assistfeatures are disposed parallel to at least one of the horizontal edgesof the plurality of features in the horizontal mask, and are disposedparallel to at least one of the vertical edges of the plurality offeatures in the vertical mask, and means for performing a featurebiasing in which at least one of the horizontal edges of the pluralityof features in the horizontal mask are adjusted such that the resultingfeature accurately reproduces the target pattern, and at least one ofthe vertical edges of the plurality of features in the vertical mask areadjusted such that the resulting feature accurately reproduces thetarget pattern.
 11. An apparatus for generating complementary masksaccording to claim 10, wherein said assist feature placement isperformed prior to said shielding.
 12. The apparatus for generatingcomplementary masks according to claim 10, wherein the amount of biasingapplying to a given feature edge is determined utilizing said model. 13.An apparatus for generating complementary masks according to claim 9,wherein prior to said shielding said horizontal mask and said verticalmask are identical to said target pattern.
 14. An apparatus forgenerating complementary masks according to claim 9, wherein said modelis determined utilizing one of a calibrated model of themultiple-exposure lithographic imaging process or a theoretical model ofthe multiple-exposure lithographic imaging process.
 15. The apparatusfor generating complementary masks according to claim 9, wherein saidshielding comprises dividing a given edge to be shielded into aplurality of segments and having the shielding applied on a segment bysegment basis.
 16. A computer program product for controlling a computercomprising a recording medium readable by the computer, means recordedon the recording medium for directing the computer to generate filescorresponding to complementary masks for use in a multiple-exposurelithographic imaging process, said generation of said files comprisingthe steps of: generating a model defining the imaging performance ofsaid multiple-exposure lithographic process, identifying a targetpattern having a plurality of features, said plurality of featurescomprising horizontal and vertical edges, generating a horizontal maskbased on said target pattern, generating a vertical mask based on saidtarget pattern, and performing a shielding step in which at least one ofthe vertical edges of the plurality of features in the target pattern isreplaced by a shield in the horizontal mask, and in which at least oneof the horizontal edges of the plurality of features in the targetpattern is replaced by a shield in the vertical mask, said shieldshaving a width which is greater that the width of the correspondingfeature in the target pattern, wherein the amount of shielding to beapplied is said shielding step is determined utilizing said model. 17.The computer program product of claim 16, wherein said generation ofsaid files further comprises: performing an assist feature placementstep in which sub-resolution assist features are disposed parallel to atleast one of the horizontal edges of the plurality of features in thehorizontal mask, and are disposed parallel to at least one of thevertical edges of the plurality of features in the vertical mask, andperforming a feature biasing step in which at least one of thehorizontal edges of the plurality of features in the horizontal mask areadjusted such that the resulting feature accurately reproduces thetarget pattern, and at least one of the vertical edges of the pluralityof features in the vertical mask are adjusted such that the resultingfeature accurately reproduces the target pattern.
 18. The computerprogram product according to claim 17, wherein the amount of biasingapplying to a given feature edge in said feature biasing step isdetermined utilizing said model.
 19. The computer program product ofclaim 16, wherein prior to said shielding step said horizontal mask andsaid vertical mask are identical to said target pattern.
 20. Thecomputer program product of claim 16, wherein said model is determinedutilizing one of a calibrated model of the multiple-exposurelithographic imaging process or a theoretical model of themultiple-exposure lithographic imaging process.
 21. The computer programproduct according to claim 16, wherein said shielding step comprisesdividing a given edge to be shielded into a plurality of segments andhaving the shielding applied on a segment by segment basis.